The technique of laser Doppler velocimetry (LDV) will be adapted to measure the velocity of red blood cells (RBC) in the human retinal vessels. This technique is noninvasive and has Already been successfully used in rabbits. Theoretical considerations imply that LDV can be performed at radiation levels lower than the maximum permissible level. A laser velocimeter suitable for use in humans is now in the final stage of construction. Data on the flow velocity of RBCs in monkeys and humans will be collected and determination of the blood flow rate will be attempted.